Merlin tumor suppressor function is regulated by PIP2-mediated dimerization

Neurofibromatosis Type 2 is an inherited disease characterized by Schwann cell tumors of cranial and peripheral nerves. The NF2 gene encodes Merlin, a member of the ERM family consisting of an N-terminal FERM domain, a central α-helical region, and a C-terminal domain. Changes in the intermolecular FERM-CTD interaction allow Merlin to transition between an open, FERM accessible conformation and a closed, FERM-inaccessible conformation, modulating Merlin activity. Merlin has been shown to dimerize, but the regulation and function Merlin dimerization is not clear. We used a nanobody based binding assay to show that Merlin dimerizes via a FERM-FERM interaction, orientated with each C-terminus close to each other. Patient derived and structural mutants show that dimerization controls interactions with specific binding partners, including HIPPO pathway components, and correlates with tumor suppressor activity. Gel filtration experiments showed that dimerization occurs after a PIP2 mediated transition from closed to open conformation monomers. This process requires the first 18 amino acids of the FERM domain and is inhibited by phosphorylation at serine 518. The discovery that active, open conformation Merlin is a dimer represents a new paradigm for Merlin function with implications for the development of therapies designed to compensate for Merlin loss.


Introduction
Neurofibromatosis Type 2 is an inherited disease characterized by benign Schwann cell tumors of cranial and peripheral nerves known as schwannomas. Schwannomas that arise in the general population also contain NF2 mutations and represent 8% of all intracranial tumors [1,2]. Targeted deletion of the Nf2 gene in mouse Schwann cells leads to schwannoma [3,4] and Nf2-null cells have impaired contact inhibition of growth in vitro [5,6]. The NF2 gene encodes Merlin, a 70-kDa member of the Ezrin-Radixin-Moesin (ERM) branch of the band 4.1 superfamily [7,8]. Merlin is predominately localized to the inner face of the plasma membrane [9,10] where it associates with lipid rafts, which are membrane microdomains enriched with a variety of signaling molecules [11]. Merlin is also a component of cell junctional complexes including adherens junctions and focal adhesions [12,13]. This is consistent with reports indicating that Merlin is a component of the HIPPO pathway, a growth suppressive signaling system which acts downstream of cell junctions and is responsible for contact inhibition of growth [14][15][16]. Merlin loss also activates other oncogenic signaling networks, including Ras-ERK, Rac, src, β-catenin, and the mTOR protein kinase complex [6,[17][18][19][20][21][22][23][24][25][26]. However, the mechanism by which these systems are activated is not fully defined. Merlin has a conserved secondary structure consisting of an N-terminal FERM domain followed by a central α-helical region that folds over itself to position the C-terminal domain (CTD) for an intramolecular interaction with the FERM domain [5,[27][28][29]. Merlin also has a unique N-terminal 20 amino acid sequence, absent in other ERM proteins, that is necessary for its tumor suppressor activity [30]. The intramolecular interaction between the Merlin CTD and FERM domains mask a large portion of the FERM domain surface area, resulting in a closed, FERM-inaccessible conformation [31]. Upon the release of the CTD from the FERM domain, Merlin transitions to an open conformation that renders the FERM domain accessible to critical interacting proteins [32]. A Merlin mutant designed to stabilize the FERM-CTD interaction results in a more closed conformation that has impaired tumor suppressor activity [32]. Conversely, a more open, FERM-accessible conformation mutant retains activity [32], demonstrating that Merlin's tumor suppressor function is facilitated by the open conformation.
Merlin is phosphorylated at serine 518 by PAK2 [33] and PKA [34]. This phosphorylation promotes a closed conformation with reduced Merlin tumor suppressor activity [32,[35][36][37]. Merlin is also regulated by binding the lipid signaling molecule phosphatidylinositol 4,5 bisphosphate (PIP 2 ) [38]. This interaction promotes Merlin localization to the plasma membrane and is necessary for tumor-suppressive activity [38]. PIP 2 binding causes Merlin to assume an open, FERM accessible conformation [39] that is mediated by a structural change in the N-terminal portion of the central α-helical domain that forces the CTD and FERM domains apart [40]. These reports show that Merlin activity is determined by conformational changes that are regulated by both phosphorylation and lipid based signaling systems. However, the specific mechanism by which open conformation Merlin interacts with binding partners to facilitate tumor suppression is not known.
The idea that Merlin forms dimers has long been established in the literature, initially by analogy to other ERM proteins and supported by the observation that its central α-helical domain may arrange itself into a coiled-coil structure [41]. Experiments using yeast twohybrid systems [42,43] and GST pulldowns with isolated Merlin N-terminal FERM and C-terminal domains [44][45][46] suggested that Merlin exists as a dimer mediated by intermolecular FERM-CTD binding. Yet, a structure derived from co-crystalized Merlin FERM and CTD domains showed that Merlin dimerizes via interactions with between adjacent FERM from the dimerization partner [47]. Other experiments using analytical ultracentrifugation explicitly ruled out dimerization [39,48]. Given the contradictory nature of the literature, there is no clear consensus regarding the function of Merlin dimerization and its relationship to conformational regulation.
To address these questions, we used a nanobody based assay to measure Merlin binding in co-transfected cell lysates and between purified Merlin proteins [13]. In addition, we use bioluminescence resonance energy transfer (BRET) to define the relative orientation of one Merlin partner interacting with another. We find that Merlin dimerizes via a FERM-FERM interaction that requires the first 18 amino acids of the FERM domain. Gel filtration experiments show that a PIP 2 mediated transition from the closed to the open conformation is required for dimerization and is inhibited by phosphorylation at serine 518. Patient derived and structural mutants show that the ability to dimerize correlates with the open conformation and tumor suppressor activity. These findings suggest novel mechanistic insights into Merlin function that integrate dimerization with conformation, phosphorylation and phosphatidylinositol signaling, providing novel mechanistic insights into Merlin function.

Merlin dimerizes via the FERM domain
To investigate Merlin dimerization, we adapted a quantitative protein binding assay that utilizes a probe consisting of purified Merlin fused to the small, bright luciferase, NanoLuc, on its C-terminus. We used a high affinity anti-GFP nanobody bound to magnetic beads to isolate wild type Merlin with GFP fused to its C-terminus. The anti-GFP nanobody enabled the purification of Merlin-GFP fusion protein to homogeneity from transfected HEK 293T cells. Binding assays were performed by incubating the Merlin-NL probe with the purified Merlin-GFP bound to anti-GFP nanobody magnetic beads (Fig 1A) [13].
To map the domains required for interaction we generated deletion mutants of the major Merlin domains fused to either GFP or NanoLuc ( Fig 1B). Purified Merlin-NL ( Fig 1C) was incubated with Merlin-GFP bound beads (Fig 1C), washed then assayed for luciferase activity. Merlin dimerization was indicated by the amount of luciferase activity normalized to the amount of GFP-Merlin on the beads relative to control. We found that wild-type Merlin-NL bound to Merlin-GFP more than 100-fold greater than control GFP ( Fig 1D). This experiment was conducted with the canonical Merlin isoform 1. Further binding experiments demonstrated that both Merlin isoforms 1 and isoform 2 dimerize with themselves and heterodimerize with each other (S1 Fig). Normalized binding data from Merlin deletion mutants revealed that the FERM domain is necessary and sufficient for dimerization ( Fig 1D). The reduced binding to the FH mutant that lacks the CTD and the small amount of binding to the HC mutant containing the CTD is evidence of a relatively weak FERM-CTD interaction that contributes to the stability of the dimer (Fig 1D). The absence of dimerization in the Merlin-H mutant shows that the central α-helical domain does not interact ( Fig 1D). This experiment demonstrated that Merlin dimerizes primarily via a FERM-FERM interaction.

Merlin dimer orientation
The luciferase substrate emission spectrum peaks 450 nm and is close enough to GFPs excitation peak at 495 nm to engage in bioluminescence resonance energy transfer (BRET), which occurs when NanoLuc and the GFP are less than 10 nm apart [49]. We took advantage of this property to determine the orientation of the Merlin dimeric complex. HEK 293T cells were co-transfected with pairs of Merlin fused to GFP or NanoLuc at either at its N-or C-terminus, Merlin-GFP:Merlin-NL, GFP-Merlin:Merlin-NL, NL-Merlin:Merlin-GFP or GFP-Merlin: NL-Merlin ( Fig 1E). The proximity of the N-and C-termini of each dimerization partner is measured by BRET, thus indicating the overall orientation of the dimer. Merlin complexes were immunoprecipitated and emission spectra from 400 nm to 600 nm were acquired. There was robust dimerization when GFP and NanoLuc are fused to the Merlin C-terminus (Merlin-GFP:Merlin-NL, Fig 1F). The BRET emission peak at 510 nm is apparent in Merlin-GFP:Merlin-NL, indicating that C-terminal GFP and NanoLuc proteins close to each other in this complex ( Fig 1G). When N-terminal and C-terminal fusion proteins were paired (NL-Merlin: Merlin-GFP or GFP-Merlin:Merlin-NL), significantly less dimerization was apparent (Fig 1F), and the 510 nm BRET peak was much reduced (Fig 1G). When both GFP and NanoLuc are Nterminal, dimerization was significantly reduced and the BRET signal was lost (GFP-Merlin: NL-Merlin, Fig 1D and 1E). Taken together, these data are consistent with the idea that Merlin forms a dimer. Dimer formation requires an uncovered N-terminus, and the complex is Merlin dimerization data quantified on a plate reader and presented as the bound luciferase normalized to 10% of the unbound input NL probe. The data is a mean of triplicate binding reactions with standard deviation and expressed as a percentage of the wild-type Merlin. Inset: an image of the light emitted from Merlin-NanoLuc bound to Merlin and Merlin deletion mutant-GFP bound magnetic beads, performed in triplicate, and dispensed into the wells of a 96-well plate (left). (E). A schematic diagram depicting the N-and C-terminal GFP fused "bait" and the N-and C-terminal fused Merlin-NanoLuc "Probe" constructs used in the BRET assays. (F). Triplicate Merlin dimerization assays for the N-and Cterminal Merlin constructs. The data is a ratio of bound to unbound NL probe, normalized to GFP fluorescence, (mean of triplicate binding reactions with standard deviation and expressed as a percentage of the peak value). (G). Emission spectrum orientated such that each C-terminus is close to the other, consistent with a FERM-FERM interaction. Since the N-terminal fusion proteins show reduced dimerization, the following experiments were conducted with the C-terminal Merlin-GFP and Merlin-NL.

Merlin dimerization correlates with the open conformation
The Merlin interaction and BRET assays strongly suggest that Merlin dimerizes. However, it remained possible that our data was the result of protein aggregation rather than dimerization. To exclude this possibility, we determined the size of purified Merlin-NL complexes using gel filtration chromatography (Fig 2A).
Fractions from the column were assayed for NanoLuc activity to determine the Merlin-NL elution profile and the approximate size was calculated in reference to the elution profile of a set of standards of known Stokes radii (S2 Fig). Merlin-NL resolved into two peaks, a major peak with a Stokes radius of approximately 5.0 nm and a minor peak of 2.1 nm (Fig 2A). There was no evidence of NanoLuc activity above this peak, ruling out Merlin-NL aggregation in these experiments. Western blot of fractions from peak 1 and peak 2 show full length protein (Fig 2B), ruling out the possibility that the 2.1 nm peak is a result of proteolytic degradation resulting in a free NanoLuc tag. Merlin that is phosphorylated at S-518 was confined to the 5.0 nm peak (Fig 2B). To determine mobility of the Merlin dimer we co-expressed Merlin-GFP and Merlin-NL and then performed gel filtration chromatography on the cell lysates. Fractions from the column were assayed for NanoLuc activity then each fraction was immunoprecipitated with anti-GFP nanobody to determine the elution profile of the Merlin-GFP:Merlin-NL complex. GFP pulldowns from the fractions show that the Merlin-GFP:Merlin-NL complex elutes as a single peak at 5.0 nm (Fig 2C). There was minimal dimer detected above this peak, further ruling out the possibility that dimerization is caused by non-specific aggregation. The total amount of NanoLuc activity pulldown from this peak was 2% of the total. This small percentage of Merlin dimer is consistent with a Kd for dimerization being significantly higher than the nanomolar concentrations at which the chromatography was performed. The remaining 98% of the NanoLuc activity represents monomeric Merlin-NL.
To determine if the 5.0 nm and 2.1 nm peaks represent different conformations of Merlin, we performed gel filtration experiments at different salt concentrations. Under isotonic conditions the ratio of the 5.0 nm to the 2.1 nm peaks was 2.6:1 ( Fig 2D). Under hypotonic conditions there was a shift from the 5.0 nm peak to the 2.1 nm peak with the ratio decreasing to 0.76:1 ( Fig 2D). Under hypertonic conditions the ratio further shifts to the 5.0 nm form, with a ratio of 3.6:1 ( Fig 2D). These data suggest that the two peaks resolved by gel filtration represent two Merlin conformations. Dimerization assays performed at different salt concentrations showed that Merlin-Merlin binding was maximal at low salt (50 mM) and lost at high salt (300 mM and 600 mM, Fig 2E and 2F), suggesting that the ability to dimerize correlates with the 2.1 nm peak. To confirm that the conformers are not an artifact of overexpression in HEK cells, we performed gel filtration experiments on cell lysates from immortalized human Schwann cells and assayed fractions for endogenous Merlin by western blot (Fig 2G). The majority of full-length endogenous Merlin elutes with peak 1 at 13 ml (5.0 nm) with a small amount in fractions that elute with peak 2 at 15 ml (2.1 nm). As with the recombinant Merlin, phospho-518 Merlin was restricted to the 5.0 nm peak. from 400 nm to 600 nm of Merlin dimerization assays normalized to the 450 nm peak. The BRET emission peak at 510 nm is indicated by the arrow. https://doi.org/10.1371/journal.pone.0281876.g001

Fig 2. Merlin conformations. (A)
Fractionation of purified Merlin-NL (inset) on an SEC 650 gel filtration column, aliquots from 0.2 ml fractions were assayed for NanoLuc activity and plotted with the elution volume. The approximate Stokes radius of the peaks was calculated relative to the mobility of standards of known molecular weight and Stokes radius. (S2 Fig). The large peak at 12.4 ml has an approximate Stokes radius of 5.0 nm and the smaller peak at 15 ml has an approximate Stokes radius of 2.1 nm. (B) SDS-PAGE with BSA standards and an aliquot of the purified Merlin-NL stained for total protein (top). Fractions composing peaks 1 and 2 were immunoblotted and probed with antibodies for either total Merlin or P-S518-Merlin (bottom). (C). Gel fractionation of lysates from cells co-transfected with Merlin-NL and Merlin-GFP. NanoLuc activity from the fractions is shown as blue circles and depicted on the right-hand y-axis. Dimers represented by NanoLuc activity from GFP pulldowns of the fractions is shown as green squares and depicted on the left-hand y-axis. (D) Gel fractionation of purified Merlin-NL at in hypotonic (50 mM NaCl), isotonic (150 mM NaCl) and hypertonic (500 mM NaCl) conditions. (E) Images of dimerization assays dimerization assays performed at 50 mM, 75 mM, 150 mM, 300 mM, and 600 mM NaCl. The GFP-NanoLuc fusion protein used to control for the effects of salt on luminescence. (F) Quantitation of the dimerization data presented as a mean of triplicate binding reactions with standard deviation and expressed as a percentage of the peak value. (G) Immunoblot assays of iHSC-1λ immortalized Schwann cell lysates fractionated by gel filtration and probed for either total Merlin (top) or P-S518-Merlin (bottom).

Merlin mutant dimerization
Dimerization was readily detected in by pulldown from lysates of cells co-transfected with wild type Merlin-GFP and Merlin-NL, confirming the presence in Merlin dimers in vivo ( Fig 3A). We then evaluated the ability of a panel of Merlin mutants to dimerize. The patient derived mutants Δ39-121, L360P and L535P all showed significantly reduced dimerization relative to wild type ( Fig 3B). However, the patient derived L64P mutant showed slightly enhanced dimerization (1.28-fold, Fig 3B); suggesting that this mutant interferes with aspects of Merlin function that are independent of dimerization. Of structural mutants, a lipid binding deficient mutant 6N had impaired dimerization (Fig 3B). A deletion of the Merlin unique N-terminal 18 amino acids (ΔN18) showed significantly impaired dimerization, 33% relative to wild type ( Fig 3C). However, GFP fused to the first 20 N-terminal amino acids (N20) failed to dimerize ( Fig 3C) indicating that Merlin's unique N-terminus is necessary but not sufficient for full dimerization. A non-phosphorylatable mutant at serine 518, S518A, had significantly enhanced dimerization whereas its phosphomimetic counterpart, S518D, had reduced dimerization ( Fig 3C). This suggests that dimerization can be regulated by phosphorylation. A mutant designed to force Merlin into a more closed conformation, Merlin-AR [32], failed to significantly dimerize relative to wild type. The more open conformation mutant, Merlin-ΔEL [32], had significantly increased dimerization ( Fig 3C).
We tested if dimerization of these Merlin mutants correlates with changes in conformers, as assessed by gel filtration. These experiments showed that the S518D mutant had a reduced 2.1 nm peak relative to both wild type and S518A (Fig 3D). This is a further correlation between the small peak and the ability to dimerize and is consistent with the western blots in Fig 1 showing that phospho-518 Merlin is restricted to the large peak. Gel filtration experiments showed that the more closed conformation Merlin-AR mutant failed to form the small 2.1 nm peak relative to both wild type and the more open conformation Merlin-ΔEL mutant ( Fig 3E). These data suggest that the 2.

Interaction of dimerization mutants with Merlin targets
We hypothesized that Merlin dimerization might affect its interaction with known target proteins. To test this, we performed co-transfection pulldown experiments with plasmids expressing wild type or mutant Merlin-NL and GFP fused to each of four known Merlin binding proteins: Angiomotin, ASPP2, Lats1 and YAP1 [13, [50][51][52]. All four proteins showed significantly reduced binding to the phosphomimetic S518D mutant relative to S518A (Fig 4A-4D).
Angiomotin bound to the constitutively closed Merlin-AR mutant but showed impaired binding to the open conformation mutant Merlin-ΔEL (Fig 4A). In contrast, ASPP2, Lats1 and YAP1, all showed either equivalent or significantly increased interaction to Merlin-ΔEL mutant relative to Merlin-AR (Fig 4B-4D). These experiments suggest that dimerization reduces the interaction of Merlin with Angiomotin and increases interaction with the key target proteins ASPP2, Lats1 and YAP1

Effect of PIP 2 and phosphorylation on dimerization
Crystallographic experiments showed that upon binding to the signaling phosphoinositide, PIP 2 , Merlin assumes an open conformation [39,40]. We confirmed this using gel filtration  (Fig 5A).
Since the open conformation correlates with dimerization, we tested if PIP 2 binding also caused enhanced dimerization. We performed binding assays with purified Merlin-GFP and Merlin-NL in increasing concentrations of PIP 2 This experiment showed increased dimerization from 6.25 to 400 μM PIP 2 -diC8, with a plateau above 200 μM (Fig 5B). Also, since the phosphomimetic S518D mutant had impaired dimerization relative to non-phosphorylatable S518A, we tested if in vitro phosphorylation by PAK2 inhibited dimerization in the presence and absence of PIP 2 -diC8. Merlin dimerization was increased in the presence of PIP 2 -diC8 by

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4.64-fold and phosphorylation by PAK2 decreased the PIP 2 effect significantly (Fig 5C). Western blots confirmed increased levels of phospho-S518 in the PAK2 treated samples (Fig 5D). Since we found that the ΔN18 N-terminal deletion mutant is deficient in dimerization and, like the 6N mutant, this region has been shown to bind lipids and to localize Merlin to the plasma membrane [30], we ran gel filtration experiments to test for PIP 2 -mediated conformational change. Although the ΔN18 mutant can assume the open conformation, it failed to increase the proportion of the open conformation in response to PIP 2 like wild type does ( Fig  5F). This suggests that a primary function for Merlin's unique N-terminus may be to facilitate the PIP 2 mediated shift to the open conformation. These results suggest a regulatory cycle in which Merlin is activated by PIP 2 , causing it to assume an open conformation that allows dimerization, leading to interactions with target proteins such as ASPP2, Lats1 and YAP1. This "active" tumor suppressing complex is then dismantled upon phosphorylation at S518 by PAK2 (Fig 6).

Discussion
We demonstrate that Merlin dimerizes via a FERM-FERM interaction, and that dimerization requires an uncovered N-terminus and the first 18 amino acids of the FERM domain. Our data is inconsistent with the prevailing model of ERM dimerization that posits an intermolecular FERM-CTD interaction between adjacent Merlin molecules [43,53]. However, there is precedent in the literature for FERM domain mediated Merlin dimerization [47]. A structure derived from co-crystalized Merlin FERM and CTD domains showed a Merlin dimer with extensive interactions between FERM domains, mediated by unfurling the F2 lobe of the

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FERM domain such that it interacts with an adjacent FERM from the dimerization partner [47]. This structure is consistent with the FERM mediated dimerization that we describe.
Our gel filtration experiments revealed that Merlin exists in two populations that represent a major peak with a Stokes radius of approximately 5.0 nm and a minor peak with a Stokes radius of 2.1 nm. The Stokes radius of 2.1 nm is smaller than predicted for Merlin and raised the possibility that the 2.1 nm peak is a degradation product of the Merlin-NanoLuc fusion protein. However, we show that full length Merlin is present in this peak, and do not see evidence of protein degradation. Stokes radius is a measure of molecular size that may be considered a combination both of molecular shape and molecular weight [54]. We calculate Stokes radius based on standards for gel filtration, which are globular proteins of known Stokes radius and molecular weight [55,56] (S2 Fig). It is likely that the elongated shape of open conformation Merlin, reduces it's Stokes radius in this assay. Our finding is consistent with data showing dramatic shifts in Stokes radius for proteins undergoing conformational changes [54].
Dimeric Merlin is detectable as a small minority of this population, roughly 2%, that coelutes with the closed monomeric conformation in gel filtration experiments. The small percentage of dimers in this population is a function of the low, nanomolar concentrations at which the binding assays are conducted. The low binding percentage at these concentrations would suggest a much higher Kd for the Merlin dimerization reaction, likely to be in the micromolar range. The behavior of the conformation mutants, Merlin-AR and Merlin-ΔEL, show that the open conformation is necessary for dimerization. The inability of Merlin-AR to dimerize is telling. Merlin-AR is an inactivating mutant that has amino acid substitutions in the extreme C-terminus, A585W and R588K, that stabilize the intramolecular FERM-CTD interaction [32] thus shifting Merlin into a more closed conformation. Gel filtration experiments show that this mutant exists predominately as a 5.0 nm peak and has a reduced 2.1 nm peak relative to wild type (Fig 3E). In contrast, Merlin-ΔEL has a deletion of the last two C-terminal amino acids, E594 and L595, that weakens the intramolecular FERM-CTD interaction leading to a more open conformation relative to wild type [32]. This more open conformation is reflected in the gel filtration data with Merlin-ΔEL showing an increased 2.1 nm peak relative the 5.0 nm peak compared to Merlin-AR and wild type Merlin. It is this increase in the ratio of the 2.1 nm peak to the 5.0 nm peak in the Merlin-ΔEL mutant and the corresponding decrease in the ratio of the 2.1 nm peak to the 5.0 nm peak in the Merlin-AR mutant that identifies the 5.0 nm peak as open conformation and the 2.1 nm peak as closed conformation.
The Merlin-ΔEL mutant retains tumor suppressor activity [32] and has enhanced dimerization relative to wild type, again linking dimerization with the open conformation and tumor suppressor function. Also, Merlin-ΔEL shows enhanced binding to three of four Merlin binding proteins, ASPP2, YAP1 and Lats1. In contrast, binding of S518D with Angiomotin, ASPP2, YAP1 and Lats1 is reduced relative to both S518A and wild type. This enhanced binding suggests that the open conformation Merlin dimers represent an activated state with increased affinity for critical target proteins.
We show that PIP 2 binding to Merlin causes a shift to the monomeric open conformation and that Merlin's unique N-terminus is necessary to shift to the open conformation in the presence of PIP 2 . Therefore, this small domain is a critical point of regulation by phospholipid second messengers. This region contains basic amino acids that are necessary for Merlin mediated growth suppression and help localize Merlin to the inner face of the plasma membrane [30] where it is a target of regulatory kinases that control Merlin stability [37,57]. These data are supported in the literature by a small angle neutron scattering study demonstrating that wild type Merlin exists in a closed conformation in the absence of PIP 2 but in an open conformation in the presence of PIP 2 [39]. This is consistent with the crystal structure of PIP 2 bound Merlin that shows a dramatic conformational change in the central α-helical domain that forces the CTD away from the FERM domain resulting in an open, FERM-accessible conformation [40]. This conformational change results in increased affinity for the Merlin binding protein Lats1 [40]. These data agree with our finding that the open conformation mutant Merlin-ΔEL has increased dimerization accompanied by enhanced binding to ASPP2, YAP1 and Lats1. Phosphorylation of Merlin at serine 518 correlates with the closed conformation and inhibits dimerization. The S518D mutant fails to adopt an open conformation upon PIP 2 binding [39]. Consistent with all these findings, we show that binding of S518D with Angiomotin, ASPP2, YAP1 and Lats1 is reduced relative to both S518A and wild type.
Together these results indicate that PIP 2 bound, dimeric Merlin complexes are responsible for binding to critical target proteins that mediate tumor suppressor function. This suggests a regulatory loop in which hypo-phosphorylated Merlin is activated by PIP 2 , causing a shift to the monomeric open conformation followed by dimerization and interaction with binding partners. This active dimeric complex is then phosphorylated on serine 518 by PAK2 leading to a closed, inactive monomeric conformation. Merlin is a mechanosensory signaling molecule that is activated in response to specific biochemical signals that originate from cell junctions in response to extracellular mechanical cues [58,59]. For instance, PIP 2 is generated at sites of Ncadherin-mediated intercellular adhesion as cells become confluent [60,61], this could provide a locally focused stimulus for Merlin to first undergo conformational change and then dimerize. Activated Merlin may then bind to specific target proteins such as ASPP2, Lats1 and YAP1 through which it regulates the HIPPO pathway [15, [62][63][64][65]. PIP 2 mediated dimerization thus provides a mechanism that allows local activation of Merlin and binding to target proteins at a specific subcellular location.
Ultimately, understanding the biochemical mechanism of Merlin function will give insight into the consequences Merlin loss and is therefore critical to the development of novel strategies to treat NF2. The model we propose for Merlin function may impact potential therapies. For example, one strategy to treat NF2 proposes to use gene therapy to replace lost Merlin function in schwannoma cells. The ability of Merlin to dimerize raises the possibility that some mutant Merlin proteins may act in a dominant negative manner, impairing the activity of ectopic wild type Merlin, potentially lessening effect of added wild type alleles. Secondly, it is clear from proximity biotinylation experiments [13] that Merlin may bind many other critical target proteins beyond the well described HIPPO pathway members Lats1, YAP1 and Angiomotin. Each novel binding partner represents a possible target for the development of drugs to treat Neurofibromatosis Type 2. It is important to identify those novel binding partners that mediate Merlin's tumor suppressor activity. It is now necessary to identify target proteins that bind to Merlin in the active, PIP 2 -bound dimeric state, thus identifying different pathways that may lead to the development of new therapies to treat NF2.

Cloning
Recombinant expression plasmids were constructed by PCR amplification of both vector and insert sequences using Q51 High-Fidelity DNA Polymerase (New England Biolabs, Ipswich, MA) followed by a Gibson assembly reaction using NEBuilder HiFi DNA Assembly Cloning Kit, then transformed into NEB-α Competent Cells (New England Biolabs, Ipswich, MA). A full listing of the primer sequences used to generate all the plasmids used in this study is provided (S4 Fig). To facilitate assembly, primers were designed with 10-15 bp extensions that are homologous to regions flanking the in-frame insertion sites in the plasmids. These sites are indicated in the maps of the vectors and shown S4 Fig. All primers were purchased from Integrated DNA Technologies, Inc. (Coralville, IA).
The expression vectors used in the study were initially derived from pEGFP-N1 and pEGFP-C1 (Takara, La Jolla, CA). A double stranded oligomer encoding StrepTag II affinity tag was cloned, in-frame, N-terminal to EGFP in pEGFP-C1 and C-terminal to EGFP in pEGFP-N1 to generate pST-EGFP and pEGFP-st, respectively. NanoLuc insert was amplified from pcDNA3.1-ccdB-Nanoluc, inserted in frame with the StrepTag II in place of EGFP in pST-EGFP and pEGFP-st to generate pST-NanoLuc and pNanoLucST. Maps and insertion sites of these plasmids are shown in S4 Fig. Full-length Merlin isoform 1 was PCR amplified form pC-Mer-Vst [66] to generate pMer-GFPst pMer-NLst, pST-GFP-Mer and pST-NL-Mer. Similarly, Angiomotin was PCR amplified from pHA-AMOT p130 and assembled into pST-EGFP to generate pST-GFP-AMOT. Merlin mutants were introduced into pMer-GFPst and pMer-NLst Via PCR amplification and Gibson assembly using the primers listed in the S4 Fig.

Cell culture
HEK-293T were used for co-transfection experiments and to purify Merlin-NL and Merlin-GFP proteins because they are readily transfectable, grow to high density and maintain SV40 ori containing plasmids in an episomal state. These features generate high yields of transfected recombinant proteins while utilizing endogenous mammalian chaperones and post-translational modification systems. The cells were purchased from ATCC (CRL-2316) grown in DMEM supplemented with 10% FBS and Pen/Strep at 37˚C 5% CO 2 . Immortalized human Schwann cell line, iHSC-1λ were a gift from Dr. Margaret Wallace (University of Florida College of Medicine, Gainesville, FL, USA) [67] also grown in DMEM supplemented with 10% FBS and Pen/Strep at 37˚C 5% CO 2 .
For small scale affinity purification, 20 μg Merlin-GFP expressing plasmid was transfected into 2 x 10 6 293T cells in a 10 cm dish using PEI, lysed as described above then affinity purified with 40 μl washed GFP-Trap_MA beads (Bulldog Bio, Portsmouth), NH) overnight at 4˚C with agitation. The beads were washed as described above and resuspended in TBST.

Direct binding assays
The GFP bound beads were resuspended in a 30-μl blocking buffer of 0.5 mg/ml BSA in TBST and incubated for 1 hour at room temp., The beads were recovered magnetically then resuspended blocking buffer plus 30 to 50 nM Merlin-NL protein and incubated at room temperature for 1 hour. Beads were then recovered, washed four times with TBST the resuspended in 25 μl TBST. Luciferase activity was measured using 25 μl NanoGlo Luciferase Substrate Buffer (Promega, Madison, WI). NanoLuc activity, GFP fluorescence and BRET spectra were measured on a FlexStation 3 microplate reader (Molecular Devices, San Jose, CA). Binding assays were performed in triplicate. Binding is calculated as bound luciferase activity normalized to a 10% aliquot of the unbound input (Bound�10% Input). These values were expressed as a mean and standard deviation relative to WT. For visual reference, after counting on the plate reader, the luminescence in the microplates was imaged with an Azure C-600 imaging system (Dublin, CA).

Co-immunoprecipitation
HEK 293T cells were co-transfected with GFP-target and Merlin-NL plasmids, lysed in 0.5 ml of TBS, 2 mM MgCl 2 , 0.5% NP-40, protease inhibitor mix. Lysates were diluted 1:2.5 with TBST then NL activity was measured. GFP fusion proteins were immunoprecipitated using GFP-Trap_MA (ChromoTek, Hauppauge, NY), recovered using a magnetic stand, and washed four times with TBS. Luciferase activity was measured and imaged as described above. To control for GFP loading, fluorescence at 485 nm excitation and 510 emission was measured on the FlexStation 3. The binding is calculated as the ratio of bound luciferase activity to unbound luciferase activity (Bound�Unbound)/GFP FLuorescence). Experiments were performed in triplicate and were expressed as a mean and standard deviation relative to WT. Statistical significance was assessed by two-way ANOVA using Prism.

Gel filtration
Gel filtration chromatography was performed on a BioRad NGC Quest 10 Plus FPLC system, equipped with a BioFrac fraction collector, controlled by ChromLab software and using a 10 x 300 mm Enrich SEC 650 gel filtration column with a 24 ml bed volume. For each run the column was equilibrated with 2 volumes of buffer at 1 ml/min, 0.25 ml sample was then injected automatically, and 10 ml buffer was run through the column before 40, 0.2 ml fractions were collected for another 8 ml. Typical running buffer was filtered and degassed PBS. For the experiments using different salt conditions, two pumps were run with Buffer A; 20 mM Tris-Cl pH 7.4, 1 mM EDTA and Buffer B; 20 mM TrisCl pH 7.4, 500 mM NaCl, 1 mM EDTA. Hypotonic buffer at 50 mM NaCl was achieved by mixing 90% Buffer A and 10% Buffer B. Isotonic buffer at 150 mM NaCl was achieved by mixing 70% Buffer A and 30% Buffer B. Hypertonic buffer at 500 mM NaCl was achieved by mixing 0% Buffer A and 100% Buffer B. Samples were passed through a 0.22 μ spin filter (Ultrafree-MD. Millipore) before injection. Samples consisted of 5-20 μg purified Merlin-NanoLuc protein in 0.25 ml column buffer. Alternatively, cleared CHAPS lysates of cells expressing Merlin-NanoLuc constructs were desalted into PBS with PD MiniTrap G-25 desalting columns (Cytiva) and concentrated to 0.25 ml by a PES spin concentrator with a 10K MWCO (Pierce). NanoLuc assays were performed with a 10 μl aliquot of fractions transferred to opaque round bottom 96 well plates and luciferase activity was measured using 10 μl NanoGlo Luciferase Assay reagent (Promega, Madison, WI) and luminescence was measured on a FlexStation 3 microplate reader (Molecular Devices, San Jose, CA). Graphs presenting luciferase activity (RLU) in each fraction or luciferase activity normalized to the activity in the peak fractions (%) relative to the elution volume in ml were prepared using Graphpad Prism 9 software.